The present invention relates to electric traction motor vehicles and, more particularly, to a system for obtaining a signal representative of the torque response of a series would direct current electric traction motor in a resistor controlled motor powered vehicle.
Transit vehicles such as subway cars are typically operated in trains, i.e., a plurality of cars are mechanically and electrically coupled to operate as a single unit. However, each car in the train has its own propulsion and control system, the control system being connected to receive operating command signals from a lead car in the train. Historically, the propulsion and control systems in each car have been essentially identical electrical systems, e.g., direct current (d-c) series wound traction motors provided motive power and a cam controller regulated the power supplied by the d-c motors. Since each car's electrical motive power system was essentially identical, an operator's command to a train of cars would result in each car responding essentially identically. Thus each car's motive power system need only be capable of supplying the power required for that car. This maximum level of required power becomes a fundamental consideration since in order to make a transit or subway car economically practical, passenger space must be maximized at the expense of available space for motive power equipment. As will be appreciated, if the motive power system on one of the cars of a train attempts to develop more acceleration than that of other cars of the train, that car will attempt to pull the entire train to the detriment of its motive power system.
In a cam controlled direct current traction motor power system, a plurality of cams mechanically coupled to a central shaft are arranged to selectively actuate a plurality of electromechanical contactors. These contactors serve to connect the d-c traction motors on the car into particular configurations. For example, in one position the armature windings of several motors may be serially connected across a d-c power source so that each motor operates on some fraction of the total source voltage. In a second position the armature winding of each motor may be connected across the d-c power source so that full source voltage is available to each motor. The cam controller also controls contactors which selectively add or subtract series resistors from the armature current path of the motors in order to regulate armature current, or rather to maintain armature current at a constant level until the d-c motor characteristics force the current to decay below the regulated level. Thus the motive power produced by the d-c motors is controlled by controlling the mode of operation of the motors rather than directly regulating output power.
When a different type of motive power system is to be utilized in selected cars of a train in which other cars of the train employ cam controlled series wound d-c motors, it is apparent that the control systems of one type of the motive power systems must be modified to respond to the type of operator's command being used by the other type of power system. For example, if an alternating current (a-c) induction motor is to be used to propel one car of a train, the control system for the a-c motor must be adapted to control the a-c motor in such a manner that the torque or power developed by the a-c powered car is equivalent to or matches the power developed by each of the d-c powered cars. A similar problem arises if the traction motor is a separately excited d-c motor or a shunt-wound d-c motor.
Accordingly, it is an object of the present invention to provide a method and apparatus for deriving a torque analog of a series wound d-c electric traction motor.